Managing Resistant Weed Populations
Over the next couple of weeks, I will be presenting information regarding weed control in an orchard system. Since herbicide resistant weeds are becoming more prevalent in the San Joaquin Valley (think Hairy Fleabane, Horseweed/marestail), it is becoming more important for growers to utilize practices that reduce the chance of herbicide resistance.
Herbicide resistance is defined as the inherited ability of a plant to survive and reproduce following an exposure to a dose of herbicide that would normally kill the wild type (Think: "We used to get good control of this weed with this herbicide..."). This is different than herbicide tolerance, which is the ability of a species to survive and reproduce a herbicide treatment with no implied selection or genetic manipulation that would make the plant tolerant (Think: "We've never gotten dependable control of this weed with this herbicide...").
Since 1980, cases of herbicide resistant weeds within California has increased from 0 to 21 reported cases. This is mainly due to the change in tillage and herbicide use practices within agriculture. In perennial crops, growers have shifted away from orchard tillage and have become more reliant on herbicide "burn downs" to control weeds in the tree rows. Glyphosate is one of the most widely used herbicides for this practice, and, until recently, has provided good control.
The formation of herbicide resistant weeds is an evolutionary process that occurs due to the application of herbicides. Since most herbicides are reliant upon a single site mode of action, it only takes a minor mutation within the plants genome to become resistant. Furthermore, the high genetic diversity of weed populations provides the opportunity for weeds to contain a mutation, thus yielding an "escape." Once the weed is unable to be controlled by the herbicide, it produces progeny that is also resistant. These seeds tend to move outward from the point of origin, causing a "hotspot" pattern within the orchard.
To control these hotspots, growers need to rotate to different herbicide modes of action. The mode of action of an herbicide is the way it alter or inhibits specific physiological or biochemical processes. This is not to be confused with active ingredient or trade name. This can be found on some herbicide labels as the WSSA or HRAC group number found below the herbicides active ingredient. Growers should also avoid the year-after-year use of the same herbicide/mode of action. In some cases, a tank mix of two products that target the escaped weed must be used.
Growers should also apply herbicides using the proper rate and at the proper timing. Herbicides give the greatest control when the weed is no larger than a silver dollar (1.5 inch diameter). As the weed grows larger than this, it becomes harder and harder to control. Growers also need to reduce the spread of weed seeds throughout their orchards. Spread may occur from harvesting equipment, spray rigs, and also natural conditions such as wind. Keeping notes of the herbicides used and escapes observed is also advised.
If you feel that you may have a resistant weed population, it is best to use any means necessary to control the weed. This may include hand weeding, herbicide tank mixes, and/or tillage. Controlling a small patch of resistant weeds is much easier than an entire orchard. It may also be of interest to contact you local farm advisor to report the incident, but the weed MUST be controlled before it goes to seed.